they keep bitcoin in their wallet for long periods of time because they expect that bitcoin price in the future, it can multiply many times the current price
Well this is certain because bitcoin now is having a lot of users because of the prediction that it will be having a price a lot more than the current price. A lot of people use bitcoin, so there is a lot of development that needs to be done and I think the more the demand, the more the price, the more it is good to save bitcoin for the future.
If we plan ti save ,then holding it for long will be the better option. Bitcoin value ruses as time passed by since adoption of new countries were starting to happen,so having btc now can be a great advantage in the future.